Spotted this at Fairburn Ings today, the black one with green head was swimming with Mallards so initially I thought it was just a manky mallard but then saw its partner which also had different colours to the usual female browns. Of note was that in comparison to Mallards they were both a little bit bigger, any ideas please

This is a descendant of the Mallard called a Cayuga Duck. It's a domestic breed known for their solid black and irridescent green feathers. It's quite normal for them to be larger in size than a standard Mallard.